The present invention relates to hydraulic or fluid-activated brake assemblies and, in particular, to safety brake assemblies for drive strings which store energy as reactive torque by reason of being under torsional strain.
Certain drive systems are subject to torque and large torsion (rotary strain) due to their length. As a result, they store a large amount of energy. When drive power to the system is interrupted, the torsion is released as back-spin and, if an uncontrolled release of torsional energy occurs, personal injury and/or property damage can result. For example, deep well submersible pumps such as progressing cavity pumps (PCPs) driven by sucker rod strings are commonly used to pump oil from deep wells. The drive strings of these submersible pumps usually have a relatively small diameter, typically of xc2xe to 1xe2x85x9 inches. Such drive strings are commonly used in wells that vary from 1,500xe2x80x2 to 6,000xe2x80x2 in depth, 3,000xe2x80x2 being a common average. Progressing cavity pumps include a stator which is attached to a production tubing at the bottom of a well and a rotor which is attached to a bottom end of the drive string. They are frequently used to pump viscous crude oil, often laden with sand or other impurities, which requires a large amount of torque. As a result, the elongated drive string is subject to considerable torsional strain. This torsional energy is stored in the elongated drive string as reactive torque. In a 3,000 foot string, as many as several hundreds of revolutions of torsion can be stored in the string if viscous sand laden crude oil is being pumped. When power is interrupted to the drive string, the torsional strain is released. Unless the release of torsion is controlled, costly and undesirable damage to equipment and/or personal injury to workmen in the vicinity can result. This is particularly true if an electric motor is used as a power source because such motors offer almost no resistance to reverse rotation when the power supply is interrupted.
Whenever drive power is interrupted to the drive string of a progressing cavity pump, the torsional strain is desirably released in a controlled fashion. Brakes which simply prevent the release of the torsion in the drive string are unsatisfactory for two reasons. First, it is preferable that in the case of an electric motor drive, the motor restart unattended when power is restored. In order to ensure a successful unattended restart, the motor must start without load. If the reactive torque is not released, the motor may not be capable of restarting and may be damaged as a result. Second, if pump repair or replacement is required any unreleased torsion in the drive string can be extremely dangerous for unaware workmen. Severe personal injury can result from the unintentional release of torsion in such drive strings.
Consequently, braking systems have been developed for control of the back-spin due to torsion in elongated drive strings. One system is a fluid brake that includes a pump engaged only when torsion is released from the drive string. The pump is used to circulate hydraulic fluid or lubricating oil from a reservoir through a restricted orifice. The resistance of the fluid created by the restriction serves to control the release of torsion. Nonetheless, this apparatus is subject to several disadvantages including excess heat production leading to potential lubricant breakdown, lack of adjustment to different load levels, and uneconomical size.
Other brake systems for controlling the release of torsion have been disclosed. Those brake systems are based on centrifugal braking principles. Examples of such braking systems are found in U.S. Pat. No. 4,216,848 which issued to Toyohisa Shiomdaira on Aug. 12, 1980; U.S. Pat. No. 4,797,075 which issued to Wallace L. Edwards et al on Jan. 10, 1989 and U.S. Pat. No. 4,993,276 which issued to Wallace L. Edwards on Feb. 19, 1991. The brakes disclosed in these patents are difficult to maintain due to internal mechanical components and require frequent maintenance when they are used to stop the reverse rotation of drive strings in a controlled manner. In addition, they are mechanically complicated and include custom-made moving parts which are costly to manufacture and expensive to keep in inventory. These brakes also tend to over-heat and burn out an oilwell progressing cavity pump system, because of the long brake cycle created by the fluid in the production tubing draining through the pump, which causes it to motor backwards and drive the drive string backwards for an extended period (up to 45 min).
A much improved brake system is known from U.S. Pat. No. 5,358,036 which issued to Robert A. R. Mills on Oct. 25, 1994. That patent discloses a relatively simple hydraulically operated disc brake mechanism which is self regulating and ensures that the torsional energy is completely dissipated in a controlled release. The brake mechanism permits axial rotation of a shaft in a first direction while retarding axial rotation of the shaft in an opposite direction. The brake mechanism includes a housing for the bearings of a downhole pump drive head which housing also functions as a reservoir for the lubricating oil for the bearings. The shaft drives a bi-directional hydraulic pump. When the shaft turns in an operating direction, the pump forces the lubricating fluid in a closed circuit through a filter and back to the reservoir. If normal operation is interrupted, and the torsion in the system is released, the bi-directional pump forces oil through a control manifold to a disc brake caliper which brakes a disc rotor mounted on the shaft. The faster the shaft turns as it releases torsion, the more braking force is applied. As the torsion is released, the reactive torque and the shaft speed drop and the brake force is correspondingly reduced until all torsion is released from the shaft, permitting safe handling and soft restart.
One problem encountered with this braking system is a build up of gas or vapour in the hydraulic circuit, especially the brake caliper of the disk brake, causing brake failure due to xe2x80x98air lockxe2x80x99. This is caused, in the first instance, by air entrainment in the lubricating oil in the bearing/gear box of the well head by agitation of the oil by the bearings, and by the gears in the case of a right angle drive. As oil is continuously drawn from the bearing/gear box during operation of the drivehead, for the purpose of lubricating the top bearing, aerated oil is being circulated continuously through the hydraulic circuit manifold. Air tends to separate from the oil and accumulate at locations with little or no flow, such as the brake lines and brake caliper wherein no oil flows during forward rotation of the drive string. Although periodic manual bleeding of the brake caliper and flushing of the brake system will largely prevent these problems in deep well PCP systems, manual intervention is expensive and completely unacceptable in the deep well pumping field, where the customer demands complete reliability without manual intervention throughout the lifetime of the pump.
In the second instance, another source of gas and/or vapour that is a common problem of disc brakes under severe duty is excessive heat generated during the braking cycle causing the hydraulic fluid to boil and release vapour. The developed gas will remain in the brake system and cause partial or complete brake failure, a condition known as vapour lock.
Thus, a disk brake system is required which overcomes these problems.
It is an object of the invention to provide a relatively simple, reliable brake system which is easy to maintain, the system permitting axial rotation of a shaft in one direction while retarding axial rotation of the shaft in the opposite direction and for substantially preventing brake fade caused by the accumulation of gas and or vapour in the hydraulic system.
It is a further object of the invention to provide a disc brake system for controllably releasing torsion stored in elongated drive strings even after extended periods of inactivation of the brake system.
It is another object of the invention to provide a means of cooling the caliper on disc brake systems and to keep the caliper and brake line clear of air, gas or vapour to prevent brake failure due to xe2x80x9cair lockxe2x80x9d and to reduce brake fade.
It is yet a further object of the invention to provide a brake system for elongated drive strings which releases torsion from the drive string in a self-regulated manner to ensure that the torsion is completely dissipated in a controlled release and which allows for the continuous selective release of accumulated gas from the brake system to reduce the occurrence of brake fade.
There is further provided in accordance with the invention a disc brake assembly for permitting axial rotation of a shaft in a first direction and retarding axial rotation of the shaft in an opposite direction, comprising:
a brake disc mounted on the shaft for rotation with the shaft;
a hydraulic fluid actuated brake mechanism adapted to engage the brake disc and retard rotation of the disc and consequently to retard rotation of the shaft;
a bi-directional pump for pumping hydraulic fluid from a reservoir, the pump being driven by the shaft;
a control manifold for directing the hydraulic fluid from the pump back to the reservoir when the shaft turns in a first direction and for directing hydraulic fluid to the brake mechanism to retard rotation of the shaft when the shaft turns in the opposite direction; and
means for preventing the accumulation of gas in the hydraulic system.
In accordance with another aspect of the invention, the hydraulic circuit further includes a venting system for continuously venting accumulated gas from at least one of the brake caliper and the brake manifold.
The means for preventing the accumulation of gas and/or vapour within the braking system, preferably includes:
a hydraulic circuit which continuously circulates oil through the brake caliper while the drivehead is in forward motion, thereby preventing accumulation of air in the brake circuit and caliper while the drivehead is in forward motion, so that the whole braking system is continuously flushed when the drivehead is operating in the forward direction.
In accordance with a further aspect of the invention there is provided a self-regulating disc brake assembly for controllably releasing torsional energy stored in an axially rotatable elastic member under torsion, comprising:
a shaft coupled with the elastic member to be axially rotatable with the elastic member in either direction;
a brake disc mounted on the shaft for rotation with the shaft;
a fluid actuated brake mechanism adapted to engage the brake disc and retard rotation of the brake disc and consequently to retard rotation of the shaft and the elastic member;
a bi-directional pump for pumping fluid from a reservoir, the pump being driven by the shaft;
a control manifold for directing the fluid from the pump through the brake mechanism back to the reservoir when the elastic member turns in a first direction and induces torsion in the member, and for directing fluid to the brake mechanism while substantially preventing flow through the brake mechanism, when the elastic member stops turning in the first direction and built-up torsion is released from the elastic member under torsion.
In accordance with the further aspect of the invention there is provided a self-regulating disc brake assembly for controllably releasing torsional energy stored in a drive string for a down hole submersible pump in a well, comprising:
a shaft coupled with the drive string and rotatable therewith;
a reservoir surrounding and rotatably supporting the shaft;
a coupling for connecting the reservoir to a wellhead assembly of the well;
a brake disc affixed to the shaft and rotatable therewith;
a fluid actuated brake caliper that straddles the brake disc for engaging the brake disc and retarding rotation thereof, and consequently for retarding rotation of the shaft and the drive string;
a bi-directional pump for pumping fluid from the reservoir, the pump being driven by a gear attached to the shaft;
a control manifold for directing the fluid from the pump through the brake caliper to the reservoir when the drive string is driven in a direction to drive the down hole pump, and for directing fluid to the brake caliper while substantially preventing flow therethrough when the drive string is no longer driven and torsion is released from the drive string to turn the shaft in an opposite direction.
In accordance with another aspect of the present invention, the assembly further includes a pressure regulating valve down-stream of the brake caliper for controlling back-spin speed during the braking cycle. The pressure regulating valve is preferably operative over the whole range of pressures possible during operation of the assembly and functions to pre-set the back-spin speed within narrow limits by controlling the pressure on the brake pads. This is achieved by the pressure regulating valve releasing a fractional stream of the brake activating fluid back to the reservoir, thereby not only keeping the system cleared of vapour, but also cooling the caliper during the brake activation cycle.
The invention therefore provides a simple self-regulating brake mechanism which safely and controllably releases torsion from an elastic member and, in particular, from elongated drive strings used to drive down hole submersible pumps such as progressing cavity pumps.
The disc brake in accordance with the invention is adapted for use in any environment where a shaft is permitted to rotate freely in one direction but must be inhibited from uncontrolled rotation in the opposite direction. The brake is a self-regulating motion arrester, not a static brake system. In other words, the faster a shaft rotates in the inhibited direction, the more braking pressure is applied up to a preselected maximum braking pressure. As the motion of the shaft is retarded, the braking pressure is accordingly reduced. When the shaft loses all momentum, the brake caliper releases the brake pads from the brake disc and the shaft is free to commence rotation in either direction.
In accordance with the preferred embodiment, the brake caliper is mounted to a reservoir which surrounds the shaft. The bi-directional pump is preferably mounted on the reservoir and exposed for ready access. The fluid in the reservoir is preferably a lubricating oil. The preferred embodiment is further provided with a fluid filter to continuously filter the lubricating oil in the reservoir.
The principle taught herein of continuously flushing air out of the brake actuating fluid and cooling the brake caliper by directing a flow of fluid through the brake, even when the brake is actuated, may be applied to any braking application.